The biggest thing I can think of is that X11 is more flexible when it comes to specifying custom resolutions; which some people use to output 240p on CRT TVs for emulation. AFAIK, there is currently no real equivalent for xrandr on any Wayland compositor.
I’ve also noticed that Grsync’s password prompt is broken on Wayland. Thankfully, it has a feature that lets you preview the command it’s about to run, so that you can copy and paste it into a terminal.
Being a KDE user on EndeavourOS, I use Wayland everyday. I switched over a few years ago for Waydroid, but now X11 isn’t even an option for me with the current release of KDE.
Sometimes you just want to have a screen showing you pretty colours on a random rotation. kde’s plasma just goes dark and kicks back to login, and kwin then crashes 9 out of 10 times on resume. Problems I never had with xscreensaver on openbox.
I like photo slideshows… never had too much trouble with the various things that do that… A lot of times I’ll just have a rotating desktop wallpaper, if I want to watch the photos I just minimize my apps.
Neither of which are drop-in replacements for screensavers. xscreensaver-gl has a whole heap more than just photo slideshows.